In a thermal bubble inkjet printing system, an inkjet printhead prints an image by ejecting ink drops through a plurality of nozzles onto a print medium, such as a sheet of paper. The nozzles are typically arranged in one or more arrays or columns such that properly sequenced ejection of ink from the nozzles causes characters and/or images to be printed on the print medium as the printhead and print medium move relative to each other. Thermal inkjet (TIJ) printheads eject fluid drops from a nozzle by passing electrical current through a heating element to generate heat and vaporize a small portion of the fluid within a firing chamber. The rapidly expanding vapor bubble forces a small fluid drop out of the firing chamber nozzle. When the heating element cools, the vapor bubble quickly collapses, drawing more fluid from a reservoir into the firing chamber in preparation for ejecting another drop from the nozzle.
During printing, heat from the heating elements affects the temperature of the thermal inkjet (TIJ) die. Thermal differences over the nozzle column area of the TIJ die have a significant influence on characteristics of the ink drops being fired from the nozzles, and can therefore have an adverse impact on the overall print quality of the printing system. For example, a higher die temperature results in a higher drop weight and drop velocity, while a lower die temperature results in a lower drop weight and velocity. Thus, variations in temperature across the die can result in variations in drop weight, velocity and shape. Differences in the drop weight, velocity and shape can have a considerable impact on the print quality. For example, drops with lower drop weight ejected from cooler areas of the die can result in printed areas on the print medium that have less ink than intended. The areas printed with less ink will appear to be lighter than other areas printed with drops of higher drop weight ejected from warmer areas of the die. Variations in drop characteristics can also adversely affect the color accuracy of the printing system. In general, print quality problems associated with inconsistent drop characteristics caused by variations in temperature across the TIJ die are referred to as light area banding (LAB), die boundary banding (DBB), and hue shift.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.